Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-06-22
2002-01-15
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06338924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photomask used in a near-field exposure system for micropattern transfer. The present invention also relates to a process for producing the photomask.
2. Description of the Related Art
Since packing densities of semiconductor chips are increasing, higher resolution is required in photolithography. In response to the requirement for high resolution, exposure wavelengths have been shortened, and illumination techniques have been improved. However, when line widths in the order of 0.1 micrometers or below are required, further improvement is needed.
In the above situations, conventionally, techniques of exposure with X-rays having shorter wavelengths or electron beams are proposed. However, those techniques have drawbacks of high equipment cost and low throughput.
For example, in distributed Bragg reflector (DBR) or distributed feedback (DFB) semiconductor laser devices, gratings are formed inside the semiconductor laser devices. In such semiconductor laser devices, sometimes grating patterns having line widths in the order of 0.1 micrometers or below are required. Generally, the gratings may be realized by high-order gratings. In the case of the high-order gratings, it is easy to form a grating because the grating pitch becomes large. However, in the high-order grating, an amount of fed-back light is reduced due to spatial diffraction light, and it is necessary to control the line-and-space ratio with high accuracy. Therefore, it is preferable to realize the gratings by first-order gratings. In the case of the first-order gratings, the required dimensions of the grating patterns are in the order of 0.1 micrometers or below. Currently, the grating patterns are formed by directly writing the grating patterns with electron beams. However, according to the conventional techniques, expensive equipment is needed, and throughput is low.
Recently, the so-called near-field exposure technology is receiving attention. The near-field exposure enables transfer of micropatterns which are finer than the diffraction limit. In the near-field exposure technique, a photomask having openings which are smaller than the wavelength of exposure light is used to expose an object such as a photoresist layer to near-field light emerging from the openings of the photomask. Since the depth and extent to which the near-field light substantially propagates are smaller than the wavelength of exposure light, the near-field light enables transfer of a micropattern having dimensions smaller than the wavelength of exposure light, to the object which is to be exposed. Due to the small depth of propagation of the near-field light, the so-called contact exposure method is used.
Conventionally, the photomasks used in the contact exposure method are produced as follows.
A shading film is formed on a surface of a mask support made of a material such as glass, which is transparent to exposure light. In the shading film, an antireflection film is added to a metal film such as a chromium film. Then, the shading film is coated with a photoresist. Next, a resist pattern with openings having smaller widths than wavelength of exposure light is formed by electron beam exposure or the like. Finally, the shading film is etched by using the resist pattern as a mask so as to produce mask openings.
Alternatively, the photomasks used in the contact exposure method may be produced as explained below with reference to
FIGS. 10A
to
10
D.
A mask support
1
made of glass or the like is provided as illustrated in FIG.
10
A. Then, the surface of the mask support
1
is coated with photoresist, and a resist pattern
2
is formed by electron beam exposure or the like, as illustrated in
FIG. 10B
, where the line widths of the resist pattern
2
are smaller than the wavelength of the exposure light. Next, a shading film
3
is formed by sputter deposition of chromium, using the resist pattern
2
as a mask, as illustrated in FIG.
10
C. Finally, the resist pattern
2
is removed so as to produce mask openings
4
at the positions from which the resist pattern
2
is removed, as illustrated in FIG.
10
D.
In the conventional photomasks produced as above, a shading film is deposited on a planner surface of a mask support, and the shading film has openings which are arranged to form a predetermined pattern. In the case where such photomasks are used in the near-field exposure, exposure light is applied through the photomask to an object such as a photoresist layer, from the opposite side to the above surface on which the shading film is formed, so that near-field light emerges through the above openings of the shading film, and the object is exposed with the near-field light when the object is placed in contact with or in proximity to the shading film.
Nevertheless, since in the conventional photomasks having the above construction, the near-field light propagates to only a small distance from the mask support, it is impossible to sufficiently thicken the shading film. Therefore, the following problems arise.
The depth to which the emerged near-field light propagates, i.e., the distance to which the near-field light propagates from the surface of the mask support in the photomask having the above construction, is at most tens of nanometers. Therefore, unless the thickness of the shading film is at most tens of nanometers, the near-field light cannot reach the object which is to be exposed, even when the object is placed in contact with or in proximity to the shading film.
However, when the thickness of the shading film is sufficiently thin, i.e., at most tens of nanometers, the shading film is prone to suffer defects such as pinholes. In addition, transmittance of light through the shading film increases with decrease in the thickness of the shading film. Therefore, when the shading film is thinned, the extinction ratio, i.e., the ratio of the amount of light transmitting through the shading film and the amount of light transmitting through the openings of the shading film becomes small, and fogging may occur in the case where the exposed object is highly sensitive.
Further, in the case of contact exposure, the shading film may be damaged by the contact with the object which is to be exposed, for example, during the operation of aligning the photomask with the object. That is, the durability of the photomask is insufficient.
However, if the above shading film is thickened to exceed tens of nanometers, the near-field light cannot reach the object to be exposed at all. Even when the thickness of the shading film is tens of nanometers, the near-field light is greatly attenuated so that the photoresist cannot receive a sufficient amount of exposure light. Thus, the upper limit of the thickness of the shading film is estimated to be 50 nm.
In addition, when the aforementioned grating pattern is produced by using the conventional photomasks and directly writing a micropattern with an electron beam, only a small margin is allowed for control of the line-and-space ratio, and the production cost becomes high.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a photomask which enables exposure of an object such as a photoresist layer to near-field light with a sufficient intensity, and allows formation of a sufficiently thick shading film having high durability and preventing fogging.
Another object of the present invention is to provide a process for producing a photomask which enables exposure of an object such as a photoresist layer to near-field light with a sufficient intensity, and allows formation of a sufficiently thick shading film having high durability and preventing fogging.
Still another object of the present invention is to provide a photomask which can be used in production of a grating pattern, allows a great margin for control of a line-and-space ratio of the grating pattern during the production of the photomask, and can be produced at a low cost.
A further object of the present invention is to provide a process for produ
Naya Masayuki
Tsuruma Isao
Fuji Photo Film Co. , Ltd.
Rosasco S.
Sughrue & Mion, PLLC
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